Method of growing nepheline crystals



Oct. 9, 1956 E. WASHKEN ET AL 2,766,105 METHOD OF GROWING NEPHELINE CRYSTALS Filed May 9, 1952 JNVENTORS EDWARD WASHKEN MARTIN J. BUERGER 2,7 66,105 METHGD CF GROWING NEPHELINE CRYSTALS Edward Washken, Belmont, and Martin J. Buerger, Lincoin, Mass., assignors to the United States of America as represented by the Secretary of the Army Application May 9, 1952, Serial No. 286,868 1 Claim. (Cl. 23-301) This invention relates to methods of clear crystals of nepheline.

It is an object of this invention to grow large, single crystals of nepheline (NaAlSiOr) suitable for piezoelectric applications.

Another object nepheline crystals growing large,

of this invention is to synthesize large from a poly-phase melt containing proper proportions of silicic acid, alumina and sodium carbonate to give the composition NaAlSiOr.

A further object of the invention is to prepare such a poly-phase melt containing an appropriate flux material.

Briefiy stated, this and other objects of the invention are obtained by preparing melts containing the proper proportions of silicic acid, alumina, and sodium carbonate plus a fiux of lithium fluoride, placing a seed crystal attached to a stirrer into the prefused melt having a temperature of about 915 C. and periodically rotating the stirrer during the crystallization period.

Although nepheline has been synthesized many times (A Comprehensive Treatise on Inorganic and Theoretical Chemistry, I. W. Mellor, vol. VI, pp. 569571) it was not until the work of Winkler (On the Synthesis of Nepheline, Helmut G. F. Winkler, Am. Min., vol. 32, pp. 131-136 (1946)) that any attempt was made to grow large crystals. The known literature, however, is of little assistance in the practical task of growing large, clear, single nepheline crystals from a poly-phase melt.

It has been found that in addition to a low temperature and low viscosity melt it is also desirable for the growth of large and clear nepheline crystals that the melt should be agitated gently during crystallization to prevent depletion of the crystal forming constituents in the neighborhood of the product.

Continuous agitation during crystallization was, however, found to be unsatisfactory in that the crystals were small and the seeds tended to dissolve. If, however, agitation was carried out only occasionally and under very gentle conditions, clear crystals of about A of an inch in diameter and more could be obtained.

The invention will become more apparent from the following description of a specific embodiment of the invention carried through in an apparatus as shown in the accompanying drawing.

The drawing shows a vertical alundum furnace muifie 1, the lower end of which is closed by a fire brick plug 2. Two graphite crucibles 3 and 4, one within the other, are placed on top of the plug 2 within the mufile 1. The upper end of the mufile 1 is closed by a fire brick cover 5 which has a central opening 6 to receive the stirring rod 7 consisting preferably of Nichrome. The stirrer 7 is contained by an upper ball bear g 8 and a lower graphite bushing 9. The ball bearing 8 is being held by a separate metal bracket 10 extending to both sides. The ends of the bracket 10 are fixed to a support 11.

The stirring rod 7 consists of a lower section 12 formed to agitate the melt. The upper section of the rod 7 is free to rotate in the bushing 9 and is driven by a gear 13. The

Minerologist, vol. 32, 1947, pages rod 7 is supported by a collar 14 resting on the bracket 10 so as to maintain the lower end of the rod 7 above the inner bottom of the crucible 4. A seed crystal 15 is attached to the lower section 12 of the rod 7 by means of a platinum wire 16.

Synthesis of nepheline may be accomplished without difficulty by melting the proper stoichiometric proportions of alumina, silicic acid and sodium carbonate together with lithium fluoride, as a mineralizer in platinum or graphite crusibles. A composition yielding excellent results consists of 86.4% nepheline and 13.6% lithium fluoride.

In preparing the melt, the weights used have to be corrected for the water and carbon dioxide which are present in the silicic acid and sodium carbonate.

The following is an example of a procedure to obtain nepheline crystals in the above described apparatus.

Approximately 400 grams of a mixture of alumina, silicic acid and sodium carbonate in stoichiometric proportions to yield nepheline, with about 14% by weight of lithium fluoride, are fused at 1200 C. for two to four hours. This treatment produces a clear homogeneous melt at the elevated temperature. The graphite crucibles 3 and 4 containing the pro-fused melt are then lowered into the vertical furnace muffie 1 to rest on the fire brick plug 2 and the temperature is raised to 1200 C. The furnace is held at this temperature for about two hours and the temperature then lowered to about 915 C. The Nichrome rod 7 with the seed crystal 15 attached to it, is then placed in position as shown in the drawing. The Nichrome rod is periodically rotated very slowly during the 24-hour crystallization period, at least once for a few minutes every hour. At the end of this time the Nichrome rod is lifted from the melt and the liquid allowed to drain from the cluster of crystals thus obtained.

The crystals grown according to the invention are large clear crystals of nepheline which may be identified as nepheline by means of X-ray patterns and optical measurements.

It will be evident for those skilled in the art of crystal growing that many variations of the above described specific embodiment are possible within the broad inven tive idea as disclosed in the specification and characterized in the claim.

What is claimed is:

The process of growing single crystals of nepheline suitable for piezoelectric applications from a melt containing nepheline and lithium fluoride which comprises placing an individual seed crystal of nepheline in said melt, maintaining the temperature of the melt at about 915 C. for a period of about 24 hours, and intermittently stirring the melt during this period for not more than a few minutes during each hour, thereby insuring the production of crystals of about Mr inch in diameter and hence suitable for said piezoelectric applications.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Dittler et al.: C. A., vol. 26, page 4277 (1932). Winkler: 0n Synthesis of Nepheline, in American 131 to 136 incl. Winkler: C. A., vol. 42, page 7123b (1948). Winkler: C. A., vol. 41, page 1716F (1947). 

